Theory Flashcards
provides the majority of the optical power
air–tear-film interface
measure the anterior corneal radius of curvature and estimate total
Standard keratometers and Placido-based
Prolate shape - what means and how much
normal cornea flattens from the center to the periphery by up to 4.00 D. is flatter nasally
changing the refractive status of the eye by 2.00 D
require altering the cornea’s thickness by less than 30 μm
altering the cornea’s thickness by less than 30 μm
changing the refractive status of the eye by 2.00 D
changing the refractive status - altering the cornea’s thickness how much
2.00 D - 30 μm
most popular wavefront analysis
Hartmann-Shack
lower-order aberrations
sphere (myopia, hyperopia) and cylinder (regular astigmatism)
higher-order aberrations
spherical aberration, coma, and trefoil
zero-order aberrations
lower-order - piston
first-order aberrations
lower-order - vertical and horizontal prisms
Myopia produces defocus
positive
hyperopia produces defocus
negative
Higher-Order Aberrations - when
pupil dilates, increase with age. particularly spherical aberration and coma, may increase after conventional surface ablation, LASIK, or RK for myopia
Higher-Order Aberrations after corrections of hyperopia
ease even more than they do in myopic eyes but in the opposite (toward negative values)
Most significant Higher-Order Aberration
Spherical aberration
Spherical aberration - pathophysiology
peripheral light rays focus in front of more central rays
Spherical aberration - symptoms signs
night myopia, halos around point images, increase depth of field but decreases contrast sensitivity
Coma and trefoil - which worse
Coma
Coma and trefoil - pathophysiology
rays at one edge of the pupil come into focus before rays at the opposite edge do. comet
Coma - when
decentered corneal grafts, keratoconus, and decentered laser ablations
Q value
degree of asphericity of the cornea
Q for spherical corneas
Q = 0
Q for prolate corneas (relatively flatter periphery)
Q <0
Q for oblate corneas (relatively steeper periphery)
Q >0
Q >0
oblate corneas (relatively steeper periphery)
Q <0
prolate corneas (relatively flatter periphery)
normal cornea Q value
–0.26
overall corneal shape including spatial thickness profiles is best expressed by
computed tomography
curvature (power) is best expressed by
Placido imaging
topographic pattern in keratoconic eyes
inferonasal or inferotemporal steepening
topographic pattern in PMD
inferior steepening, which is most dramatic between the 4 and 8 o’clock positions, with superior flattening. “crab-claw” shape
oblate
steeper peripherally than centrally
significant change in prescription for glasses or contact lenses
greater than 0.50 D in either sphere or cylinder within the past year
